Wherefore, my beloved, as ye have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling.
Note 1 at Php 2:12: Here, the Philippians were encouraged by Paul to be faithful in his absence as they had been in his presence. Notice that he did not tell them to work for their salvation; rather, he told them to work out their salvation (see note 2 at this verse).
Note 2 at Php 2:12: Salvation is what God did for us through Jesus Christ. It is the gift of God (see note 4 at Ro 6:23) that can only be received by faith. When we put our faith in Jesus as our Lord, God puts salvation and all its blessings in us (Php 2:13), but we have to work it out.
The phrase "work out" was translated from the Greek verb "KATERGAZOMAI," and according to Wuest's Word Studies from the Greek New Testament, it means "'to carry out to the goal, to carry to its ultimate conclusion.' We say, 'The student worked out a problem in arithmetic.' That is, he carried the problem to its ultimate conclusion. This is the way it is used here. The Philippians are exhorted to carry their salvation to its ultimate conclusion, namely, Christlikeness."
Php 2:13 reveals there is a divine enablement that wills and is able to perform God's bidding in our lives, but there is an effort on our part too. We have to work it out. This work needs to be understood in the light of the labor spoken of in Heb 4. We are to cease from trust in ourselves and rest in the Lord. That takes effort.
Note 3 at Php 2:12: The Amplified Bible translates the last part of this verse as "work out (cultivate, carry out to the goal, and fully complete) your own salvation with reverence and awe and trembling (self-distrust, with serious caution, tenderness of conscience, watchfulness against temptation, timidly shrinking from whatever might offend God and discredit the name of Christ)."